1. Field of the Invention
The present invention relates generally to ink-jet hard copy methods and apparatus and, in particular, to the use of an optical sensor to monitor drop deposition characteristics which can be related to ink depletion in pen.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIG. 1 depicts an ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral printer, 101. For convenience in describing the art and the present invention, all types of ink-jet hard copy apparatus are hereinafter referred to as "printers," all types, sizes, and compositions of print media are also referred to as "paper," and all compositions of colorants are referred to as "ink;" no limitation on the scope of the invention is intended nor should any be implied.
A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller (usually a microprocessor or application specific integrated circuit ("ASIC") controlled printed circuit board) 102 connected by appropriate cabling to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions for conventional or general purpose microprocessors or with ASIC's. Cut-sheet print media 105, loaded by the end-user onto an input tray 120, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station, or "print zone," 123 where graphical images or alphanumeric text are rendered onto adjacently positioned paper. A carriage 109, mounted on a slider 111, scans the print zone 123 (stationary paper wide ink-jet writing instruments are also known in the art and may be employed with the present invention). An encoder system 113 is provided for keeping track of the position of the carriage 109 at any given time. Individual ink-jet pens 115X are mounted in the carriage 109. Reusable printhead systems are fluidically coupled by tubing 119 to replaceable or refillable ink reservoirs 117X (generally, in a full color system, inks for the subtractive primary colors, cyan (X=C), yellow (X=Y), magenta (X=M) and true black (X=K) are provided; ink fixer (X=F) solutions are also sometimes provided). Each pen 115X operates using an internal back pressure regulator for allowing transfer of ink from a respective reservoir 117X while maintaining the appropriate back-pressure needed for the operation of each printhead of each pen. Note, it is also known in the art to provide replaceable ink jet cartridges which have a self-contained ink reservoir and back-pressure regulating mechanism. Once a printed page is completed, the print medium is ejected onto an output tray 121. As indicated by the labeled arrows, the scanning axis is referred to as the "x-axis," the paper transport path as the "y-axis," and the printhead firing direction as the "z-axis."
A simplistic schematic of a swath-scanning ink-jet pen 115X is shown in FIG. 2 (Prior Art). The body 200 of the pen 115X generally contains an ink accumulator and regulator 202 mechanism. The internal accumulator and regulator 202 has a fluidic coupling 204 for the off-axis ink reservoir 117X (FIG. 1 only). The printhead 206 element includes an appropriate electrical connector 208 (such as a tape automated bonding flex tape) for transmitting signals to and from the printhead. Columns of nozzles 210 form an addressable firing array 212. The typical state of the art scanning pen printhead may have two or more columns with more than one-hundred nozzles per column. In a thermal inkjet pen 115X, the drop generator mechanism includes a heater resistor subjacent each nozzle 210 which superheats locally chambered ink to a cavitation point such that an ink bubble's expansion and collapse ejects a droplet from the associated nozzle 210. In commercially available products, piezoelectric and wave generating element techniques are also used to fire the ink drops. Other ink-jet writing instruments are known in the art; some, for example, are structured as page-wide arrays. Degradation, or complete failure of the drop generator elements, cause drop volume variation, trajectory error, or misprints, referred to generically as "artifacts," and thus affect print quality.
Closed-loop ink-jet printing sensors enable a printer to monitor variable operational attributes and make appropriate adjustments automatically or to provide signals indicative of operational conditions to the end-user. One important attribute is ink level detection. In large format printers--such as for the printing of poster art--running out of ink in the middle of a print job is an inefficient and costly problem. Moreover, printing errors, also known as "artifacts," occur because of drop volume variation.
The most prevalent method of ink level sensing is to drop count. The print job data stream is provided into subsets for each of the primary colors. Therefore, tracking the number of times each nozzle is fired should theoretically account for ink consumption; the number of drops multiplied times the nominal drop volume subtracted from the fill level for each ink equals the volume of remaining ink. However, in fact, the drop volume may vary in absolute value by only one picoliter. With a nominal drop volume of approximately five picoliters in the state of the art, this is a relatively large percentage variation. Multiplied by the millions drops fired by a pen to create a single print swath, the error translates into large variations in the volume of ink consumed. Furthermore, the fill volume of each pen or reservoir also varies in accordance with manufacturing tolerance specifications.
One prior art solution is to have drop counters operate by firing each drop through a beam of light. A detector determines the percentage of the nozzles in which the ejected drop occludes the light beam. A decrease in the percent of nozzles which fire droplets blocking the light signifies the onset of an empty reservoir. However, the percent of missing nozzles can only be correlated to the ink remaining over the last few percentage of ink remaining. As drop volume decreases, both in design and due to reservoir depletion, more sophisticated optical interrupters must be employed to ensure accuracy, increasing manufacturing costs.
Another mechanism for ink level sensing is to provide a mechanical or fluidically controlled gauge on the print cartridge or reservoir. This requires regular monitoring by the end-user. Consequently, it is of less value than an automated system.
Another mechanism is to provide an electrical trigger which sends a signal to the end-user when the ink reaches a certain minimum level. These inductive sensors are a relatively expensive addition to the manufacturing cost of a simple ink supply tank.
There is a need for ink level sensing which is more accurate and less costly than state of the art modalities.